Phase Linear Diagnosis Help requested

[jppatti]

I have recently acquired a Phase Linear Dual 500 Series 2 amplifier, and am in need of some assistance in diagnosing a potentially dead channel.

Upon receiving the amp, it was missing the mains fuse, along with the fuses for channel A. The channel B fuses were present, but blown. I popped the top off the amp to see if there was anything obviously blown up or black, but all I could see that looks out of the ordinary was a dark spot under the large resistors at the far right if the driver board, and the traces for these look fine.

I got the proper mains fuse (ABC 15) and decided to try to power the power supply up without the channel power fuses installed. With the amp on a bulb limiter, it came up fine, so I took the limiter out and let it start up, again without the channels power hooked up. It started without issue, and the main caps are holding at solid +/-122V DC.

I then checked for continuity on the channel power supply fuse holders (output side of the fuse holders) and found that I was reading continuity at about 3 ohms on channel A between positive and negative. Channel B does not read any continuity between positive and negative.

At this point, I think I may have a bad output transistor or two in the channel A, but I wanted to see if anyone who is experienced with these amps could provide some insight as to the best way of going about testing for and fixing the problem.

Thanks

EDIT: I pulled the amp apart a bit further and got a really good look at the driver board, and it actually appears that R105 and R205 (2.7k ohm) are blown up, so I have replaced them with new resistors from my stash.

 

[mbz]

You need to work out why R105 and R205 blew. Obviously excessive current but what caused that? failed transistor???
Not clear where you measured the 3 ohms (to GND?) 3 ohms does suggest a short/problems.

The pheonix forum are the experts in all things PL if AK help is a bit sparse
https://forums.phxaudiotape.com/

 

[jppatti]

Thanks a lot for the response.

After probing around some more, all of the transistors in the bad channel are reading shorted, so it seems I need to pull each transistor and test them individually.

You are absolutely right that I need to figure out what caused the over-current in the first place. I am assuming that whatever is wrong on the driver board is what caused one or more of the outputs to fail.

Thanks for the heads up on the other forum. I will try my luck over there as well.